This invention relates generally to light sources. More particularly, it relates to electric light sources.
Highly efficient and economical multi-watt red, green, and blue lasers are desirable for display applications. Unfortunately, existing semiconductor lasers are generally not available at power levels and wavelengths suitable for display applications. Displays for consumer applications require reliable multi-watt sources that can be manufactured in high volumes. Although many factors influence manufacturing costs, some of the most significant are the number of parts and alignment steps, tight tolerances, and the cost of components.
The wavelengths of currently available lasers can be converted to those required for displays using nonlinear optics. Nonlinear optics has been used to produce wavelengths throughout the visible spectrum, over a wide range of powers, with optical-to-optical efficiencies well in excess of 50%. However, the relatively low nonlinear coefficient of available materials requires resonant or mode-locked frequency conversion schemes that are incompatible with the economics of displays. Alternatively, nonlinear waveguides may be used, but these have limited power-handling capability. To meet the needs of display applications, a new class of nonlinear optical materials employing quasi-phase-matching is required.
To reduce the number of parts and alignment steps, a bulk single-pass configuration for second harmonic generation (SHG) using one infrared semiconductor laser and one nonlinear crystal, is preferred over a resonant design. Generally, the semiconductor laser and nonlinear crystal are the costliest components in this source. Semiconductor laser bar prices have declined at approximately 30%/year since 1985. Single-emitter multi-watt semiconductor lasers are expected to follow a price-volume relationship similar to that of laser bars. The cost of the nonlinear crystal is also strongly tied to volume and the stability of the fabrication technology. Consequently, it is more reasonable to modify existing nonlinear optical materials than to develop new materials. Lithium niobate (LiNbO3), often referred to as xe2x80x9cthe silicon of nonlinear optics,xe2x80x9d is an excellent material for SHG for two reasons. First, LiNbO3 is already produced at a volume of 40 tons per year for consumer applications (cellular phones and televisions) using a very stable fabrication technology. Second, LiNbO3 is transparent from 350 nm to 5000 nm, providing low loss for both the fundamental and harmonic for visible light generation. Finally, LiNbO3 has nonlinear coefficients for visible light generation among the highest of all inorganic materials.
While LiNbO3 is an attractive material because of its status as a commodity material, the only component of its nonlinear tensor large enough to satisfy the requirements of display applications is d33, having a value of 25.2 pm/volt. While dispersion prevents direct access to the full d33 coefficient, quasi-phase-matching (QPM) can provide up to 64% of the full nonlinearity, or 16 pm/volt, making LiNbO3 a very strong candidate for display applications that use QPM.
Essentially QPM is a technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion. In QPM, two waves having different phase velocities shift xcfx80 out of phase relative to one another over a distance called the coherence length. The sign of the nonlinear coefficient reverses every coherence length, causing the locally generated harmonic field to transfer power to the harmonic beam. By compensating for phase-velocity mismatch in this way, all elements of a crystal""s nonlinear tensor can be accessed throughout the entire transparency range.
Two other potential materials in which QPM has been demonstrated for visible light generation are LiTaO3 and KTiOPO4 (KTP). LiTaO3 has a normalized room temperature conversion efficiency of 0.83%/(watt.cm), below that required for bulk single-pass 1064 nm SHG. However, for 852 nm SHG, LiTaO3 has a normalized conversion efficiency of 1.8%/(watt.cm) and would be suitable for that application. KTP has a normalized conversion efficiency for 1064 nm SHG of 1.7%/(watt.cm). To achieve 25% single-pass conversion efficiency of a one watt fundamental, a crystal of 3.6-cm length is required; however, the maximum crystal length in production is 3 cm. For 852 nm SHG, KTP""s normalized conversion efficiency is 4.1%/(watt.cm), and would be a strong candidate for that application.
Various approaches have been studied to create QPM structures, including use of rotationally twinned crystals, stacking of alternately oriented thin plates, and growth of periodic domain structures in ferroelectrics. For waveguides where QPM is required only at the surface of the crystal, periodic annihilation of the nonlinear coefficient and periodic domain inversion by dopant indiffusion in ferroelectrics have been employed. Periodic domain structures can be formed in ferroelectrics by applying an electric field using lithographically defined periodic electrodes. Yamada, et al. xe2x80x9cFirst-order quasi-phased matched LiNbO3waveguide periodically poled by applying an external field for efficient blue second-harmonic generation,xe2x80x9d Applied Physics Letters 62 (5), 435-6 (1993) and S. Matsumoto et al. xe2x80x9cQuasi-phase matched second harmonic generation of blue light in electrically periodically poled lithium tantalite waveguidesxe2x80x9d Electronics Letters Vol. 27, Issue 22 P. 2040-2042, 1991 were the first to report a demonstration of this approach. This last technique is referred to as electric field periodic poling, and is now often referred to simply as periodic poling. (xe2x80x9cPolingxe2x80x9d refers to the process whereby the spontaneous polarization of a ferroelectric crystal can be reversed under the influence of a sufficiently large electric field. In this application, the term xe2x80x9celectric field periodic polingxe2x80x9d will be used to differentiate from other periodic poling techniques.)
In both electric field periodic poling and waveguides, lithographic techniques are used to assure the periodicity of the QPM structure. The fabrication of masks for lithography typically employs interferometric feedback control. This type of control can limit the positional error of any feature over the dimension of the mask to less than a quarter-wavelength of radiation, e.g. 0.16 xcexcm for He-Ne. For a 5-cm-long 5-xcexcm-period grating, this amounts to a maximum period fluctuation of 6 parts in 1 million, resulting in a negligible reduction in conversion efficiency. The ability to define QPM structures with lithographic precision created an opportunity in nonlinear optics to fabricate devices with interaction lengths not possible using non-lithographic techniques.
Previous periodic poling techniques have produced 50-mm-long, 0.5-mm-thick periodically poled LiNbO3 (PPLN) with a 29.75-xcexcm period. For visible light generation using PPLN, many applications would benefit from electric field periodic poling technology capable of producing domain periods below 15 xcexcm in devices at least 1.0 cm long. Domain periods between 6 xcexcm and 7 xcexcm are required for green light generation, and blue light generation requires domain periods between 4 xcexcm and 5 xcexcm. The longest prior PPLN devices for visible light had a period of 4.6 xcexcm, were 6 mm long, and a thickness of 200 xcexcm. Domain pattern quality had decreased as the period was reduced.
Therefore, a need exists in the art for an electric field poling process that accounts for the dependence of domain quality on period and thickness to produce shorter domain periods.
Accordingly, it is a primary object of the present invention to provide a model of the electric field periodic poling process in LiNbO3 that predicts poling outcomes and is useful as a design tool. It is a further object to provide an optimized poling waveform. It is an additional object to provide a means for fabricating non-linear crystals that are quasi-phase-matched over their entire length.
These objects and advantages are attained by a novel method for fabricating a periodically poled structure from a ferroelectric substrate having an electrode structure and an insulator structure. The method produces an electric field within the substrate by applying a voltage waveform to the electrode structure. The waveform raises the electric field magnitude to a level substantially greater than that required to reverse domains within the substrate. The waveform then lowers the voltage such that the electric field has a value at which a domain wall velocity is most sensitive to changes in the field. The waveform maintains the electric field value until a current through the substrate drops substantially. The electric field is typically maintained for a time greater than a relaxation time of the substrate material. Alternatively, the electric field is maintained for a time t determined by t=x/v, where x is a domain wall half-width and v is a domain wall velocity. Subsequently, the waveform may lower the voltage to zero. Preferably the voltage is reduced at rate slower than a rate of relaxation of domains within the substrate.
The method may enhance periodic poling by providing annealing the substrate in oxygen to increase an electrical resistance of the surface. Oxygen may also be provided to the surface of the substrate by coating the surface with a spin-on glass and baking the substrate to form the insulator structure.